Catalytic steam reforming of low carbon atom hydrocarbons (methane, natural gas, naphtha) and the water gas shift are the main routes for producing H.sub.2 and synthesis gas, a H.sub.2, CO mixture, for use as fuel and chemical synthesis. The CO.sub.2 -hydrocarbon reforming route and the combined steam and CO.sub.2 hydrocarbon reforming route are also possible routes for production of H.sub.2 and synthesis gas but their catalysis is much more sensitive to carbon deposition and further developments are needed in both catalysis and reactor design for large scale process operation.
The above are mostly endothermic processes (with the exception of the water gas shift which is slightly exothermic), and the necessary heat load into the reactor to run the reactions at the desired temperature range can be provided by burning hydrocarbons from waste and flue gases.
The usual catalysts for the hydrocarbon steam and CO.sub.2 reforming reactions and the combined one, are nickel (Ni) based alloys enriched with earth metals to prevent coke deposition during reaction and are usually supported on oxides of alumina (Al.sub.2 O.sub.3), titania (TiO.sub.2), silica (SiO.sub.2), and less often zirconia (ZrO.sub.2); Ruthenium (Ru), Rhodium (Rh), Palladium (Pd), Platinum (Pt) metal structures mixed with earth metals can also be used. For the water gas shift reaction used catalysts are based on iron (Fe), zinc (Zn), chromium (Cr), copper (Cu), nickel (Ni), cobalt (Co) compositions enriched with earth metals which are supported on similar to the above supports.
The produced H.sub.2 from the above reactions is usually separated in a consecutive pressure swing adsorption unit; the CO.sub.2 is commonly separated by using absorption in alkaloamine solutions and molecular sieve type adsorbents.
The participating reactions are given below: EQU CH.sub.4 +H.sub.2 O(g)=CO+3H.sub.2 (.DELTA.H.sub.298 =206.1 KJ/mol), methane-steam reforming (1) EQU CO+H.sub.2 O(g)=CO.sub.2 +H.sub.2 (.DELTA.H.sub.298 =-41.15 KJ/mol), water gas shift (2) EQU CH.sub.4 +CO.sub.2 =2CO+2H.sub.2 (.DELTA.H.sub.298 =247.3 KJ/mol), methane-CO2 reforming (3)
Membrane permeators offer selective removal of specific gas products in a consecutive separation step. Membranes can be integrated within the reaction vessel itself to make permreactors (membrane reactors) which integrate reaction and separation in a single unit operation. Previous studies involve metal and ceramic membranes used in various types of high temperature catalytic reactions and processes related to hydrocarbon processing and conversion such as steam reforming, water gas shift and alkane dehydrogenations. Polymer membranes have been mostly used in gas and liquid separations at low to intermediate temperatures; as example, they have been used in purification of natural gas from CO.sub.2, H.sub.2 S, N.sub.2, halogens compounds. Membrane process designs can offer increased reactant conversions, product yields and product selectivities over other reaction-separation schemes such as reaction combined with adsorption, absorption, cryogenic separation, distillation and other separations. The selective product removal with the membrane can shift the equilibrium conversion to the product side and according to the mass conservation equation of a chemical reaction, the reactant conversion and subsequently the product yield can surpass the respective ones at equilibrium. Higher conversions and yields in membrane processes can make the processes to operate suitably at lower temperatures and increase their thermal efficiency and the life cycle of the catalyst and reactor wall materials, thereby reducing capital and operation costs.
U.S. Pat. No. 5,183,482 reports on separation of gas mixtures such as separation of H.sub.2 from CO.sub.2, H.sub.2 from N.sub.2, H.sub.2 from He, by using inorganic aluminum alkoxide (ceramic) membranes. U.S. Pat. No. 5,229,102 reports on applications of catalytic alumina ceramic membrane reactors for steam hydrocarbon reforming with H.sub.2 separation. U.S. Pat. No. 4,826,599 reports on methods for producing polymer membranes with permselective properties for separation of various gas mixtures. Earlier communications have reported on preparation and related gas separation applications of organic polymer and inorganic membranes and membrane based reactors.
In the aforementioned reports the objective is separation of a key product component (usually H.sub.2) directly out of the reaction zone (in a membrane reactor case) or in a subsequent membrane process after the reactor (in a membrane permeator case). It is objective of this invention to provide hydrocarbon reforming and water gas shift processes involving organic polymer, organic polymer-inorganic supported membranes, and inorganic membranes which separate simultaneously H.sub.2 and CO.sub.2 gases from other compounds. The described membrane processes offer mass and thermal advantages over other reaction-separation schemes. As example, the proposed membrane processes can be successfully applied when the initial reactors (reforming or gas shift reactors) operate at low conversion reaction conditions and substantial amounts of reactants, which are not separated through the membranes, are recycled into initial reactor inlet or used in consecutive reactors in the same or different reactions. Moreover, direct utilization of the separated H.sub.2 and CO.sub.2 mixture can be in methanol synthesis and as feed in molten carbonate fuel cells for power generation; also after the CO.sub.2 removal, the utilization of H.sub.2 in chemical synthesis or as fuel are additional objectives of the invention. Utilization of the thermal load of the exiting from the reactor gases to generate steam to be used internally in steam reforming and water gas shift reactors, in an autothermic reactor mode, is an additional advantage of the invented processes.